The present invention relates to a method and apparatus for determining the location of submarine and subterranean reservoirs. The invention is particularly suitable for determining the boundaries of a reservoir which contains hydrocarbons and whose approximate location is known.
Currently, the most widely used techniques for geological surveying, particularly in submarine situations, are seismic methods. These seismic techniques are capable of revealing the structure of the subterranean strata with some accuracy. However, a seismic survey can not provide the information in real time. It is necessary with seismic techniques to gather seismic information and then analyse the results before taking any further readings.
It is therefore an object of the invention to provide a system for locating the boundaries of a subterranean reservoir in real time.
It has been appreciated by the present applicants that while the seismic properties of hydrocarbon filled strata and water-filled strata do not differ significantly, their electromagnetic resistivities do differ. Thus, by using an electromagnetic surveying method, these differences can be exploited.
Electromagnetic surveying techniques in themselves are known. However, they are not widely used in practice. In general, the reservoirs of interest are about 1 km or more below the seabed. In order to carry out electromagnetic surveying as a stand alone technique in these conditions, with any reasonable degree of resolution, short wavelengths are normally necessary. Unfortunately, such short wavelengths suffer from very high attenuation in a conductive medium as water filled strata. Long wavelengths do normally not provide adequate resolution. For these reasons, seismic techniques have been preferred.
However, while longer wavelengths applied by electromagnetic techniques cannot provide sufficient information to provide an accurate indication of the boundaries of the various strata, if the geological structure is already known, they can be used to determine the nature of a particular identified formation, if the possibilities for the nature of that formation have significantly differing electric characteristics. The resolution is not particularly important and so longer wavelengths which do not suffer from excessive attenuation can be employed.
The resistivity of seawater is about 0.3 ohm-m and that of the overburden beneath the seabed would typically be from 0.3 to 4 ohm-m, for example about 2 ohm-m. However, the resistivity of an H/C reservoir is likely to be about 20-300 ohm-m. Typically, therefore, the resistivity of a hydrocarbon-bearing formation will be 20 to 300 times greater than that of a water-bearing formation. This large difference can be exploited using the techniques of the present invention.
The electrical resistivity of a hydrocarbon reservoir normally is far higher than the surrounding material (overburden and underburden). EM-waves attenuate more rapidly, and travel slower inside a low resistivity medium, compared to a high resistivity medium. Consequently, hydrocarbon reservoir will attenuate EM-waves less, compared to a lower resistivity overburden. Furthermore, the EM-wave speed will be higher inside the reservoir.
A refracted EM wave behaves differently, depending on the nature of the stratum in which it is propagated. In particular, the propagation losses in hydrocarbon stratum are much lower than in a water-bearing stratum while the speed of propagation is much higher. Thus, when an hydrocarbon-bearing reservoir is present, and an EM field is applied, a strong and rapidly propagated refracted wave can be detected.
Thus, an electric dipole transmitter antenna on or close to the sea floor induces electromagnetic EM fields and currents in the sea water and in the subsurface strata. In the sea water, the EM-fields are strongly attenuated due to the high conductivity in the saline environment, whereas the subsurface strata with less conductivity has less attenuation. If the frequency is low enough (in the order of 1 Hz), the EM-energy is able to penetrate deep into the subsurface, and deeply buried geological layers having higher electrical resistivity than the overburden (as e.g. a hydrocarbon filled reservoir) will affect the EM-waves. Depending on the angle of incidence and state of polarisation, an EM wave incident upon a high resistive layer may excite a ducted (guided) wave mode in the layer. The ducted mode is propagated laterally along the layer and leaks energy back to the overburden and receivers positioned on the sea floor. In the present application, such a wave mode is referred to as a xe2x80x9crefracted wavexe2x80x9d. When the hydrocarbon filled layer ends, the wave guide also ends and the refracted wave dies out. This edge effect is diagnostic for the reservoir boundaries.
The distance between the EM source and a receiver is referred to as the offset. Due to the fact that a refracted wave in a hydrocarbon-bearing formation will be less attenuated than a direct wave in seawater (or in the overburden), for any given H/C bearing formation, there will be a critical offset at which the refracted wave and the direct wave will have the same magnitude. This may typically be about two to three times greater than the shortest distance from the source (or receiver to the H/C bearing formation). Thus, when the offset is greater than the critical offset, the EM waves that are refracted into, and guided through the reservoir, will pay a major contribution to the received signal. The received signal will be of greater magnitude and arrive earlier (i.e. have smaller phase) compared to the case where there is no HC reservoir. Due to the critical angle, the receivers are rather sensitive to edge effects and the resolution will be far better than one wave length. The boundaries can therefore be identified with quite high accuracy.
When using time domain pulses, the signals through the overburden and the refracted waves in the reservoir will arrive at different times. This effect might be used to identify the extent of a reservoir. However, pulses suffer from strong dispersion in the conductive medium.
If the offset between the transmitter and receiver is significantly greater than three times the depth of the reservoir from the seabed (i.e. the thickness of the overburden), it will be appreciated that the attenuation of the refracted wave will often be less than that of direct wave and any reflected wave. The reason for this is the fact that the path of the refracted wave will be effectively distance from the transmitter down to the reservoir i.e. the thickness of the overburden, plus the offset along the reservoir, plus the distance from the reservoir up to the receivers i.e. once again the thickness of the overburden.
According to one aspect of the present invention, there is provided, a method for locating the boundary of a hydrocarbon-containing reservoir in subterranean strata, which comprises: deploying an electromagnetic transmitter; deploying an electromagnetic receiver; applying an electromagnetic (EM) field to the strata using the transmitter; detecting the EM wave field response using the receiver; analysing the response to determine the presence or absence of a hydrocarbon-containing reservoir; moving the receiver to another location; and repeating the procedure; in which method, the path taken by the receiver in moving from location to location is determined by the signal characteristics of previously detected EM wave field responses. When nearing the reservoir boundary, the edge effects, being a drop in detected magnitudes, will be identified. By measuring in different directions, the reservoir is delineated.